1. Field
The present invention relates to bicycle wheels constructed of composite materials and to associated methods of manufacture. More particularly, the invention relates to bicycle wheels made from fibers impregnated with synthetic resins, and methods for fabricating such wheels.
2. State of the Art
Typically, bicycle wheels have been constructed to include an outer rim shaped to accept a pneumatic tire, joined with a central hub structure by a multiplicity of metal wire spokes. As the interest in bicycle riding has increased, both at the amateur and professional sports level and at the recreation and exercise level for others, the interest in the use of plastics to reduce the weight and the aerodynamic drag of the bicycle components has kept pace. Composite structures exhibit high strength to weight ratio, making them desirable for human powered vehicles such as bicycles. These composite materials are composed of fibers impregnated with a synthetic resin. The fibers, for example, could be carbon, Kevlar.RTM. or thermoplastics such as polyamides or polyethylene. The synthetic resin matrix could be either of thermosets such as epoxies, polyesters or the thermoplastics, including many polymers. The composite material forms used in the prior art have typically been woven cloth and/or unidirectional tape.
Several bicycle frame producers have developed light-weight frames with good strength, but the degree of improvement has in general not occurred with bicycle wheels. However, the disc wheel may be considered an exception, wherein the wheel comprises two molded inwardly facing concave composite discs sandwiching a core of honeycomb material or plastic foam, in turn attached to the hub and a tire receiving rim. The resulting wheel structure is aerodynamically efficient in straight on winds, but weighs more than conventional wire spoked wheels.
Bicycles wheels must be stiff both laterally and radially. Low lateral stiffness potentially causes the wheel to flex excessively, increasing bicycle control problems, in the worst case increasing the likelihood of rim collapse. Radial stiffness is required to resist the vertical forces, including shock loads, without excessive deflection. Wheel flexing taxes the rider's energy, as well as decreases the pleasure of riding. Thus, the need exists for lightweight stiff wheels which also minimize resistance to side winds. It seems mandatory to replace the disc wheel with a wheel allowing side winds to pass more or less freely therethrough. In other words, a spoked wheel composite plastic construction offers definite benefit.
Several composite bicycle designs have been proposed since the inception of the disc wheel to improve wheel weight and lateral aerodynamics. In U.S. Pat. No. 4,919,490, lateral aerodynamics are improved by use of a three spoked composite wheel. The spokes have a cross sectional shape of an airfoil. The entire wheel structure is handlaid with composite cloth, and is resin injection molded with an outermost metal tire receiving rim. The resulting wheel has improved lateral aerodynamics, but is still relatively heavy and expensive even in comparison with conventional wheels. U.S. Pat. No. 5,246,275 discloses a similar, but three spoked, composite wheel structure. In this case, the composite structure is hollow, without any internal foam or honeycomb. The hollow structured wheel is lighter, but at the expense of costly hand lay up manufacturing methods. In U.S. Pat. No. 5,104,199, a composite four spoked wheel is disclosed which essentially comprises a disc wheel design without the core material and with large portions of the discs cut away to form the spokes. This design has good lateral aerodynamics, light weight and a simplified method of manufacture. However, the lateral stiffness of the wheel is compromised. Each spoke comprises a pair of thin walled composite plates with little buckling load resistance, making it difficult to achieve acceptable radial and lateral stiffness.
Typically, these prior art wheels are all constructed by hand laying up of plies of fiber reinforced plastic cloth, either upon a male mandrel or upon the inside surface of a pair of split mold halves. The laid up structure is subsequently compressed by internal or external pressure respectively. Heat is applied in some instances, depending on the selected resin matrix. The use of laid up cloth materials makes it very difficult to select fiber orientation to most efficiently and directly resist the spoke and wheel loads. The use of hand laid cloth materials makes it impossible to achieve a low cost method of manufacture.
Accordingly, all the prior art design approaches leave something to be desired structurally. Still needed is a novel bicycle wheel construction which utilizes the strength of the composite reinforcing fibers of plastic in the most efficient manner to reduce bicycle and wheel weight as much as possible.